1. Field of the Disclosure
The invention relates to a method for fabricating volume diffractive elements in photo-thermo refractive glass. More particularly, the invention relates to holographic optical elements and specifically, to volume Bragg gratings (VBG) fabricated in doped photo-thermo refractive (PTR) glasses.
2. Discussion of Prior Art
The diffractive optical elements including VBG and UV light induced refractive index structures, which are fabricated in photothermorefractive glass, have been recently widely accepted in optoelectronics. VBGs, for example, are the effective optical solution to stabilizing the output wavelength from a commercial laser diode.
A typical method for recording VBGs involves a prism-based interferometer which has a thin elongated plate of photosensitive glass coupled to the prism's face. See U.S. Pat. No. 7,391,703 which is incorporated herein in its entirety by reference. The prism is made from material transparent at a given wavelength. The exposure of the prism to an incident light wave leads to the recordation of VBGs along the plate's surface.
A few inconveniences may be encountered during the manufacturing process of VBGs using the above-discussed interferometric approach. For example, the intensity of the exposure should be uniform in order to have uniform index change and refractive index modulation along and across the plate. This, however, may be technologically challenging. Also challenging may be the spatial stability between the beam and the plate coupled to the prism, which likewise is required for a reproducible fabrication of gratings. The coupling between the prism and plate may be sensitive to misalignment and requires a good mechanical stability. A further inconvenience presented by this method may include dicing the plate so to receive individual VBGs since it is being done by cutting the plate transversely to the longitudinal direction of the grating fringes, when the angle between the grating planes and the glass surface is desired.
A further method of volume grating fabrication using side interferometric recording allows for the fabrication of large and thick volume holograms as taught U.S. Pat. No. 5,491,570 which is incorporated herein in its entirety by reference. This approach, like the one previously discussed, may not be efficient in mass production because of the difficulty to maintain the desired alignment between the components. In addition, this method does not teach teaching dicing a slab of glass because the final product includes large, thick volume holograms, not small VBGs.
Method for manufacturing fiber Bragg gratings utilizes a simpler, more efficient approach than those discussed above. A grating is typically imprinted in the core of an optical fiber using a silica glass grating phase mask, as disclosed in U.S. Pat. No. 5,367,588 which is incorporated herein in its entirety by reference. “Laser irradiation of the phase mask with ultraviolet light at normal incidence imprints into the optical fiber core the interference pattern created by the phase mask.” Id. Structurally, an apparatus for implementing the method is configured with a stationary light source radiating light having a Gaussian profile which is incident upon the mask that, in turn, is juxtaposed with a fiber.
A few obvious advantages of using a phase mask include, but not limited to, the use of low coherence excimer lasers for grating fabrication and reliable and reproducible length of gratings. These advantages are critical for efficient mass production. Perhaps one of possible undesirable consequences associated with the fiber grating production process stems from the stationary light source which is typically a laser with a long coherence wavelength.
In general, any lengths of fiber can be irradiated as long as it does not exceed the dimension of the used mask. However, the radiation emitted by the single mode stationary laser has a substantially Gaussian profile characterized by a high intensity field along the laser axis and smaller field intensities gradually changing as the wings of the profile run away from its central, axial region. As a consequence, the mask is not uniformly exposed to light which leads to the variation of grating parameters such as reflectivity and central wavelength. The field uniformity issue was resolved by a laser displaceable relative to a mask. See U.S. Pat. No. 5,066,133 incorporated herein in its entirety by reference.
A need, therefore, exists for a method of fabricating VBGs in an efficient manner used in mass production.
Another need exists for the method of fabricating VBGs which is characterized by the light (UV) exposure uniformity.
Still another need exists for the method of fabricating VBGs which is characterized by the reproducibility of the grating's parameters, particularly, grating period so important in mass production.